The present invention relates to acid testers and more particularly to a system for measuring the concentration of hydrofluoric and nitric acid in solution with each other.
Pickling baths such as used in the manufacture of stainless steel include in many instances the combination of hydrofluoric acid with nitric or other strong acids having free hydrogen ions. Inasmuch as both acids include free hydrogen ions, it has in the past been difficult, if not impossible, to measure the concentration of the constituent acids of the bath by conventional means. This is true even though acid testing meters utilizing electrodes have been commercially available and can be used to measure either hydrofluoric or nitric acid concentrations when only one of the acids is present in solution.
Efforts have been made to develop a titration process for the measurement of hydrofluoric and nitric acid concentrations in stainless steel pickling baths but the resultant process is very cumbersome and has not been commercially practical or successful.
More recently, a hydrogen ion selective membrane electrode of the general type disclosed in U.S. Reissue Pat. No. 24,222 has been utilized in combination with a fluoride electrode and a reference electrode common to both electrodes to provide voltages representative of the hydrogen and fluoride ion concentration in a pickling bath including both hydrofluoric and nitric acids. By utilizing a plurality of solutions of known concentrations of hydrofluoric and nitric acids and plotting a graph of the known concentrations versus the voltage measured by the hydrogen selective electrode and the fluoride ion selective electrode, curves are plotted which correspond to the respective concentrations of hydrofluoric and nitric acids for the measured electrode voltages.
An instrument has been developed incorporating such electrodes and graphical representations utilizing antilog amplifiers and suitable numerical control circuits for subtraction, multiplication and addition for processing the measured electrode voltages and providing a digital display of the acid concentration for pickling baths employing both hydrofluoric and nitric acids. Although such a system represents a significant improvement over the cumbersome titration process and permits the direct readout of acid concentrations utilizing a three electrode system, the instrument developed lacks the flexibility desired for expanding the system to other acid combinations and represents electrically an elementary approach to the solution of the problem.
As a result of the state of the art at this time, most users of mixed acid baths have not attempted to continuously monitor hydrofluoric and nitric acid concentrations but merely replace the acid bath or refresh the bath periodically or do so when an inspection of the resultant product indicates the pickling bath needs changing.